As described in, for example, Japanese Patent Publication No. 2004-005783 (“Patent Document 1”), a sealed structure of a conventional magnetic disk unit has been a structure built by mounting various components on a base plate fabricated by metal die casting, press working, and so forth, and securing a cover fabricated by metal die casting, press working, and so forth to the base plate with screws or the like through the intermediary of packing.
Incidentally, as a magnetic disk unit has since come to be mounted in a portable terminal, and so forth, further miniaturization, lower cost, lighter weight, lower profile, higher density, and so forth are now required of the magnetic disk unit.
However, with a fabrication method relying on the conventional technology described above, since the base plate and the cover of the magnetic disk unit are formed of a metal material, there is a problem that material cost increases. Further, since the base plate and the cover are formed of the metal material, there is also a problem that specific gravity of the magnetic disk unit becomes higher, causing a weight of the magnetic disk unit to become greater.
Further, when the base plate and the cover are formed by die casting, unless respective parts are designed so as to enable molten metal to flow therethrough, shrinkage cavities can locally occur at certain times. Accordingly, configuration in wall-thickness of the cover as well as the base plate, and layout of respective gates thereof need to be taken into consideration, however, because it is difficult under such constraints to extensively provide portions of those parts, extremely small in thickness, on the order of not more than 0.3 mm, it is not possible to reduce the wall-thickness any further, so that there arises a problem that it is difficult to attain the lower profile of the magnetic disk unit.
Then, when the base plate and the cover are formed by press working, those parts are formed by drawing of one and the same sheet material, so that fabrication is carried out under a constraint that all the parts have to be designed to have an identical wall-thickness. In consequence, if a wall-thickness (plate thickness) is small, it is not possible to obtain sufficient rigidity between respective fixed parts of the unit, for example, between a spindle motor and a voice coil motor for driving a carriage, so that if vibration, impact, and so forth are applied from outside when data is written to, or read from a magnetic disk, this will cause a problem that an operation to write or read the data cannot accurately be executed. On the contrary, if the wall-thickness is large, this will raise a problem of such a large wall-thickness interfering with further miniaturization, lower cost, lighter weight, lower profile, and higher density.